I have found out the Solution.
Read the error clearly it suggests about twentyfourteen theme.
When you see this error.
Do the following.
Go to your cpanel or ftp where theme files are located.
Rename your theme folder / directory Twenty Fourteen to "TwentyFourteen2"
Now come back you your wordpress and go to yoursite.com/wp-admin
you will be able to access your site now, and change the theme to TwentyTwelve and click activate.
Update remaining things and then you are good to go!
Thanks :)